Slide-bead coating is a process well known in the art. It entails flowing a liquid layer or layers down an inclined slide surface to an efflux end, or lip, positioned a short distance from a moving substrate. The liquid forms a bridge, or bead, in the gap between the lip and the moving substrate. The moving substrate carries away liquid from the liquid inventory in the bead in the same layered structure established on the slide. See, for example, Russell, et al., U.S. Pat. Nos. 2,761,791 and 2,761,419.
Customarily, the slide bead coating process is initiated through a sequence of steps. As depicted in FIG. 1, the flow of the coating solutions, 1 and 2, is established with the coating roll, 7, and coating head assembly, represented as 3 and 4, (and any associated attachments) far enough apart to allow the coating solution to flow as a moving film of liquid over the face of the coating plate 3 and into vacuum chamber, 14. It is drained from chamber 14 through tube 16 to sump 17. The coating head assembly, 3 and 4, and coating roll, 7, are then moved close enough to establish flow across the gap, 5, between the coating head and the substrate, 6, as depicted in FIG. 2.
At the instant of coating initiation and for a time thereafter, a residual liquid film, 20, covers the coater face, 21, from the base of the newly formed bead, 18, down into the vacuum chamber, 14, as illustrated in FIG. 3. In the vicinity of the bead, 18, strong bulk viscous and interfacial extensional forces tend to pull liquid up from the residual film, 20, into the bead, 18, and thereby onto the substrate surface, 6. Further down the coater face, 21, the liquid film continues to flow toward and into the chamber, 14, thinning the residual film, 20. This thinning occurs most rapidly in the vicinity of the bead, 18. Eventually, the liquid film at the thinnest point either ruptures or dries depending on whether or not the liquid wets the coater face, 21. In either case a stationary wetting line, or static contact line, 22, is formed as shown in FIG. 4.
In the time following coating initiation and before static contact line formation, the liquid from the residual film that is pulled up into the bead, 18, may contain particles or agglomerates from deposits originally located below the bead on the coater face, 21. These particles and agglomerates subsequently interact with the flow in the bead, either directly interfering with the bead internal flow pattern or indirectly interfering with the uniformity of the coating flow envelope by contacting the lower bead meniscus. Defects in the film such as a variation in the coated thickness across the substrate, are generated by these types of interference. Such coating defects, often called streaks by those practicing the coating art, may render the resulting material unusable for the intended application. In other cases, the transient liquid film flowing into the base of the meniscus may be irregular and can consequently cause nonuniformities in the coating during the liquid film transient.
The transient liquid film flow in which the coating is either immediately or subsequently vulnerable to streaks may last as long as ten seconds depending on the liquid viscosity, substrate speed, coating and pre-coating flow rates and coater face geometry. If the coater face, 21, diverges from the substrate by approximately 15 degrees or more, the transient duration will be long, typically greater than 5 seconds. If the coater face, 21, is approximately parallel with the substrate surface, 6, the transient will be terminated very quickly, typically in less than about one second. Unfortunately, such quick termination also leads to streak defects because the liquid film, 20, dries or ruptures thus forming an irregular static contact line prior to the bead lower meniscus terminus reaching its equilibrium position for steady-state operation.
Various technologies have been proposed to avoid the occurrence of the streak defects. The technology disclosed by Hitaka and Takemasa, U.S. Pat. No. 4,440,811, modifies the coater lip region to include a notch whereby the bead contact line is preferentially located along the notch tip. However, the proposed configuration is expensive to fabricate to the precision required, and in practice the notch promotes deposits and settling from the flowing material and is difficult to clean.
Japanese Patent Publication No. 48-4371 discloses use of a land inclined with respect to the substrate tangent so as to locate the wetting line at the sharp coating lip. This configuration allows the sharp lip region to be excessively vulnerable to mechanical damage in the form of a crack or scratch that would, in-turn, result in streaks. To avoid this problem Jackson, in U.S. Pat. No. 3,928,678, discloses the technology of rounding or bevelling the tip edge of the lip to increase the mechanical robustness of the lip tip. The configuration additionally positions the bead static contact line away from the lip. However, no dimensions or orientations are disclosed whereby the bead static contact line can be preferentially and advantageously positioned at the lower edge of the bevel. As stated by Hitaka et al in U.S. Pat. No. 4,440,811 in reference to using such a bevel: ". . . it was difficult to hold the end of the beads at a fixed place or to restore the said end to the original state."
A more successful approach is disclosed in commonly assigned copending patent application Ser. No. 07/823,696 (filed concurrently herewith under Assignee's Case No. IM-0430). In that case, the lip region is configured with a short upper lip land that intersects a lower lip surface at an angle large enough to preferentially locate the wetting line at the line of intersection. Although this technology successfully avoids static contact line irregularities and subsequent streaks, it is still vulnerable to occasional streaks caused by particles occasionally washing into the intersection line region from residual film flow during start-up.